In my network there is no domain, so we use UVNC with a basic password authentication.We just begun setting up a domain with Samba4, but it will take a long time to join all the clients to it, so I would like to know if's possibile to configure UVNC servers (with new MS logon or some configuration tweak) to accept connections only from Samba server user accounts, even if their are still in workgroup.

This is to enforce security and logging of VNC connections across network.

A workgroup-joined computer cannot authenticate domain accounts because it has no trust in that domain. You can use local accounts on all computers regardless of domain membership to make uVNC security consistent across them all.